1. Field of the Invention
The present invention relates to the field of cell phones and other portable equipment using a USB bus connector.
2. Prior Art
The USB bus has five bus connections, namely VBUS (power), two data lines D+ and D—, an ID (identification) line and ground. In a cell phone application using a MiniB or micro-B USB connector on the phone, an accessory connected to the connector may be identified by the phone by checking the value of a resistor coupled between the ID line and ground line in the accessory. In that regard, there is a published specification CEA-936A describing the use of a Mini-B or Micro-B USB connection for other accessories, but it only allows 2 wires (D+ and D− data lines) for signals. Consequently a Mini-B or Micro-B USB connection in accordance with CEA-936 A specification cannot accommodate the combination of a microphone and a stereo headset.
A typical microphone detection circuit detects the bias voltage of the attached microphone. As shown in FIG. 1, a comparator is used to check the voltage across the microphone JFET preamplifier with a typical 2.2K pull up resistor. This pulls the ID line to the supply voltage when a microphone is not connected. With a microphone connected, the voltage across the microphone is about 1.18 volts, and the output of the comparator changes state due to the inverting input being biased at 2.27 volts. However the bias voltage of the microphone varies due to JFET characteristics and makes reusing the microphone connection for detecting other accessories difficult and unreliable.